High dielectric materials have many uses in semiconductor devices. Mainly, high dielectric materials are used to form dielectric layers of capacitors and gate insulators of transistors.
Conventionally, layers of SiO2, having a dielectric constant of 3.85, have been used to form gate insulators. Recently, however, much effort has been spent looking for new materials that have better characteristics than SiO2 with which to form gate insulators and the like. As a result of such efforts, for example, Ta2O5 has been discovered. Ta2O5 is a metal oxide having a dielectric constant of 25. It is highly rated as a gate insulator and may advantageously substitute for SiO2 because of its improved characteristics.
Unfortunately, however, Ta2O5 contains impurities as well. Ta2O5 layers always contain elements that substitute for Ta. Such elements can cause current leakage, and thus impair the dielectric effect. These impurities can enter the dielectric material through oxygen vacancies. Oxygen vacancies occur due to instabilities in the Ta2O5 layer itself. The amount of the oxygen vacancies depends on the concentration of the respective elements and the coherence between elements. Nevertheless, it is impossible to remove the oxygen vacancies completely. Thus, to prevent current leakage, one must attempt to stabilize the stoichiometry of the Ta2O5 through oxidation processes. Such processes change the oxidation state of the impurities that would otherwise substitute for Ta elements within the Ta2O5 layer. Accordingly, Ta2O5 gate insulator formation processes becomes complicated owing to a low temperature oxidation processes such as plasma annealing and UV-O3 annealing preformed in-situ or ex-situ.
Where tantalum ethylate (Ta(OC2H5)5) is used as a precursor to Ta2O5, current leakage also occurs via the carbon elements inside of the Ta2O5 layer. That is, carbon elements, such as unbounded carbon, CH4 and C2H4 generated during the deposition process of the Ta2O5 layer and left in the Ta2O5 layer deteriorate the quality of the Ta2O5 layer and degrade its dielectric characteristics.
In order to obtain high dielectric Ta2O5 layer, one should crystallize the amorphous Ta2O5 layer using a high temperature oxidation process. During this process, however, a Si—Tax—Ox compound oxide layer is changed into SiO2 having a low dielectric constant. As a result, the Ta2O5 layer loses the merit of metal-based oxide layer having a high dielectric constant.
Alternately, a low temperature oxidation process may be introduced to prevent the Si—Tax—Ox compound from converting to SiO2. Such processes do not crystallize the amorphous Ta2O5 layer, however,making it difficult to obtain a metal based Ta2O5 layer having a high dielectric constant.